A vehicle ventilation system, particularly for an automobile, may be called upon to provide an array of temperatures and air flows during an operating cycle. Ventilation systems are therefore equipped with controls for temperature, and air flow which may controlled by adjusting blower speed. Temperature can be regulated by controlling air flow from a heat source either at the source or a point along the duct work of the system. Ordinarily, opening and closing the mouth of a duct with a door is a simple task, but space can be a limiting factor in vehicle that prevents use of an ordinary door. A space saving door is required to reduce space required for opening and closing. In the past, space saving doors have used hinges, bellows, film belts and slider members where one member is hollow to receive another member slidably therein forming a variable length door. While these devices have worked, they required extra motors, belts and pulleys and were not always simple to manufacture. Accordingly, it will be appreciated that it would be highly desirable to have a variable length air duct door whose length changes to accomplish air flow control in the limited space available in a vehicle without employing extra belts or pulleys and which is simple to manufacture.